Implement pairwise sum

[wolfv]

When NumPy's sum function is called, it is actually a pairwise sum implementation, which is supposed to be more stable: https://en.wikipedia.org/wiki/Pairwise_summation

[wolfv]

Actually, according to this, the pairwise sum doesn't seem to perform much better in terms of accuracy

https://github.com/nschloe/accupy

which questions if we should implement it. I vote for no (except we want to be 100% true to numpy).

[iamthebot]

Assuming multithreaded single-axis reductions are implemented (eg; sum) you already get many of the benefits of pairwise summation (each thread computes a partial sum and then the partial sums are combined).

[wolfv]

https://github.com/rust-ndarray/ndarray/pull/577/files